Many inks, adhesives and other curable coatings comprise free radical based or cationic formulations which may be photo-cured by exposure to light, typically ultraviolet (UV) or short wavelength visible radiation. Applications include curing of large area coatings, adhesive curing, as well as the print processes such as inkjet printing. Curing uniformity is critical for many large area photo-induced curing processes.
For example, UV curable free radical based photo-reactive inks have increased in popularity for use in inkjet printers. Such inks are usually jetted on top of a substrate with one or more layers and pass under a UV or visible light source for curing. Photo-initiators in the ink formulation are activated by photons, e.g. UV light energy, to create free radicals, which are highly reactive with other components in the ink such as monomers and oligomers. The resulting free-radical initiated polymerization or cross-linking reaction results in a solidified ink layer. In a typical inkjet application, the irradiation period occurs in a fraction of a second or less. When the ink leaves the irradiation zone, polymerization or solidification may continue, which is referred to as dark reaction. Such printers can print on flexible substrates such as polyvinyl chloride (PVC) and other flexible polymeric materials, and rigid substrates such as metal, wood and plastics.
Typical parameters to assess a UV inkjet printer include print quality, print speed, print width, type of substrate, reliability, for example. Among these, the combination of print quality and speed is often considered most challenging. Besides the print heads, which controls how ink droplets are jetted, UV light sources used for curing play an important role in the influence of print quality and speed. Traditional UV light sources used in inkjet printers are typically mercury (Hg) arc lamps and another class of Hg lamp, a microwave or electrode-less bulb, although other gas discharge lamps, may also be used. These lamps provide high enough power to cure most types of inks at print speeds used in the industry to date and are used in low cost and high cost printer systems. However, the amount of heat irradiated from gas discharge lamps is usually very high, which places constraints on system design. Overheating may cause operational and maintenance problems. Excessive heat also limits the ability of inkjet printers to print on some heat sensitive substrates. However, if the lamp power is lowered to avoid deleterious heating effects, there may be a trade off, e.g. in lower print quality and speed, or curing may not be achieved at all.
In recent years, solid state light emitting devices (LEDs), such as light emitting diodes, have been developed as alternative light sources for industrial processes, such as photoreactive or photo-initiated processes, e.g. photo-curing of inks, adhesives and other coatings. LEDs are more energy efficient than traditional gas discharge lamps. Solid-state light sources may also be preferred for environmental reasons, as well as longer lifetime. UV LEDs have attracted a lot of attention because they generate much less heat and consume much less power than gas discharge lamps, for the same light output.
UV LED sources commonly used in the inkjet industry have lines or arrays of a large number of LEDs packed closely to each other so that jetted ink layers receive continuous irradiation. Many of the applications of UV LED sources in inkjet printers use arrays of packaged LED devices or chip-on-board die with direct illumination so that light is spread out or diffused. Examples of such arrangements are described in US Patent Publication No. US2007/0013757 by Mimaki and in U.S. Pat. No. 7,137,696 to CON-TROL-CURE. These arrangements may have difficulty in achieving a uniform beam profile and an intensity that is high enough for good print quality for some applications. More densely packed LED chips may be provided to achieve high intensity, however water cooling may then be required which adds to the system complexity and cost. An upper limit for irradiance of a densely packed LED array may also be limited by the packaging or mechanical housing of individual die that determines the minimum spacing between die.
Modular LEDs arrays are disclosed for example in U.S. Pat. No. 6,683,421 to Kennedy, entitled “Addressable semiconductor array light source for localized radiation delivery”.
Copending U.S. patent application Ser. No. 61/161,281 by Sheng Peng and Guomao Yang, entitled “Distributed Light Sources for Photo-reactive curing” discloses a light source comprising a plurality of linear LED arrays for producing a beam profile with high intensity irradiated regions and a dark region in between. This type of irradiation pattern takes advantage of intervals of both photoreactions and dark reactions.
With available LED arrays, for high speed printing applications using very short exposure times of the substrate to be cured, an array formed from individually packaged LED die, or a single row of LED die, may not provide sufficient intensity, and it is necessary to provide a higher density array to provide a line, a band, multiple lines, or multiple bands of illumination of higher intensity. High density arrays for module assemblies are available, for example in linear LED arrays or rectangular or square LED arrays.
For wide format printing it may be required to provide a linear array light source of say 35 cm or more wide, made up of a number of linear array modules, which are each about 2-5 cm long, for example, that are abutted to form a long linear array.
A problem encountered with available LED arrays, is that a linear array of a single row of individual LED die, for example, may be closely spaced along the row, but, because of the packaging and housing surrounding a linear array of individual die, when two arrays are abutted, the spacing between adjacent LED die, on different modules may be about 7 mm, for example, i.e. much larger than the spacing between dies on the same module (see FIG. 1B). Similarly, high density square arrays are known, for example, as manufactured Enfis™, which use 5×5 diode arrays. These arrays may be arranged in groups to provide larger area high intensity arrays, for example as described in the above-mentioned U.S. Pat. No. 6,683,421(Kennedy). In another example, the LEDZero™ product, manufactured by Integration Technologies™, uses multiple LED arrays in a linear arrangement to provide a wide band of illumination.
In a typical square 5×5 LED array, for example, each LED die may be 1×1 mm2, so that these arrays may be about 7 mm×7 mm. However, the arrays are fabricated on a substrate, which has a 1 mm to 2 mm edge that surrounds the individual die. The die may be hermetically sealed within the package, which required a minimum wall thickness around the LEDs, to provide a good seal. When these LED arrays are abutted, there is a gap or spacing between adjacent groups of LEDs, which may be 2 mm to 4 mm (see FIG. 1C). Thus, because of the thickness of the wall of the package or housing of available high density LED arrays and/or other design/assembly needs, there is a gap or spacing between LED die on neighbouring modules when they are abutted in a module assembly. This means that there is uniform intensity along the length or width of the each module, but there is a dip in intensity/irradiance in the region where each module abuts, which tends to cause a banding effect in the substrate being cured.
U.S. Pat. No. 6,450,664 to Stockeryale shows a modular LED array assembly, which provides a denser arrangement of LEDs near ends of the assembly to provide a uniform irradiance profile with a sharper edge, i.e. approaching a rectangular or “top hat” function. Similarly, U.S. Pat. No. 6,380,962 to Miyazaki provides an arrangement to provide an irradiance profile with a sharper edge using wider light source near ends of a linear light source. However, neither of these applications addresses the problem of providing a more uniform irradiance profile where two modules abut. In fact a sharp profile may exacerbate edge effects in modular arrays, i.e. creating a more marked discontinuity or dip in irradiance caused by the spacing where two modules abut, depending on the size of the gap or separation between LED elements due to the mechanical housing.
In another example, disclosed in U.S. Pat. No. 7,175,712, LEDs are arranged in staggered rows, and LED arrays are also staggered to provide a more uniform irradiance. However, because of the thickness of the substrate and packaging surrounding the array, this arrangement also does not overcome a discontinuity in irradiance around edges of the arrays, or where two arrays abut. There is a need for a high intensity modular LED array, which provides uniform irradiance over a large area, and avoids problems with a discontinuity in irradiance when two or more LED modules are abutted.
Not only is uniform irradiance desirable for effective curing, but also, since curing may also be wavelength dependent, improved UV LED array light sources that provide some degree of wavelength control are also desirable. Formulations and colours of inks and coatings vary from supplier to supplier, and for different application requirements. They have different curing requirements, including requiring different wavelengths of UV light. It is known that the appearance of a cured ink or coating film may be dependent on the wavelength of UV irradiation used for curing, and use of different wavelengths, e.g. 365 nm and 400 nm, may result in different surface finishes, e.g. matte or glossy surfaces. Typically, adding some 365 nm radiation to a 400 nm based LED source improves surface curing. It may also be desirable to provide flexibility for use of different wavelengths of UV because absorption, reflection of UV light by different colours of inks and coatings, as well as the substrate materials, may be very different.
The light output or intensity may be monitored by one or more photodiodes in each module of the array. Large arrays require a larger number of photodiodes to monitor intensity of each module and ensure uniform irradiance from module to module. These photodetectors also take up space in the array, and add to expense. It is also desirable to provide improved monitoring and control of the intensity from each module to provide uniform irradiance from module to module.
Thus, further improvements in modular LED array light sources are needed, particularly, for large area photo-curing or other photo-initiated processes.